Japan Geoscience Union Meeting 2016

Presentation information


Symbol B (Biogeosciences) » B-PT Paleontology

[B-PT08] Evolution of Chemosynthetic Ecosystem in Earth History

Thu. May 26, 2016 3:30 PM - 4:45 PM Poster Hall (International Exhibition Hall HALL6)

Convener:*Robert Jenkins(School of Natural System, College of Science and Engineering, Kanazawa University), Hiromi WATANABE(Japan Agency for Marine-Earth Science and Technology), Takami Nobuhara(Science Education (Geology), Faculty of Education, Shizuoka University), Ryuichi Majima(Faculty of Education and Human Sciences, Yokohama National University)

3:30 PM - 4:45 PM

[BPT08-P02] Reconstruction of seepage history in the Eocene Poronai Formation, Hokkaido, Japan

*Takami Nobuhara1, Yuriko Shirotori1, Shintaro Hitomi1, Toyoho Ishimura2, Akira Ijiri3 (1.Science Education (Geology), Faculty of Education, Shizuoka University, 2.Department of Chemistry and Material Engineering, National Institute of Technology, Ibaraki College, 3.Kochi Institute for Cores Sample Research, JAMSTEC)

Keywords:chemosynthetic, Vesicomyidae, methane seepage, Poronai Formation, Eocene

The Eocene Poronai Formation, central Hokkaido, is known to mark the oldest fossil records of vesicomyid bivalves in Japan. This study made an outcrop mapping of lithology, mode of fossils occurrence, and stable isotopic analyses to make clear the seep-habitats in initial stages of vesicomyid diversification through the Cenozoic age.
A series of irregular-shaped seep-carbonate rocks, about 1-2 m in diameter, vertically piled up in the outcrop section of massive siltstone. The carbonate rocks and the surroundings yield abundant shells of chemosynthetic bivalves, Hubertschenkia ezoensis and Conchocele bisecta. Their modes of fossil occurrences are divided into two types, 1) shell-concentrated lens and 2) sporadic patches and scattering, which alternate each other in the vertical section. The shell-concentrated lenses are mainly recognized in siltstone, whereas the scattering type is encompassed in the carbonate bodies.
It is noteworthy that all the shell-concentrated lenses were associated with calcitic concretions partly containing fluidized texture just below them. The fluidized part consists of mixture of 1) white-colored detrital micrite depleted in δ13C (–38.05 to -22.91‰), 2) gray-colored micrite not depleted in δ13C (4.87 to 9.01‰) and 3) black-colored sparitic cements with widely ranging values of δ13C (-42.09 to 1.88‰). Detail lithological mapping show that such fluidized texture tends to be formed avoiding shell-rich part. These suggest that the alternating pattern of two modes of fossil occurrences was controlled by intermittent fluidizing events as follows. Sporadic biofacies was formed under the diffusive phase resulting in gradual rise of pore-water pressure. Over the critical point of pore-water pressure, fluidization was triggered in unconsolidated parts not rich in buried shell remains, which acted as a nucleus of precursor concretions. Such local fluidization caused a focused flow fostering shell-concentrated mode.